Rotors of a rotating electric machine can have an active part with a rotor core and rotor fingers defining slots for rotor windings.
At the axial ends of the rotor fingers, supports for the rotor windings (support blocks) are provided; these supports are fixed to the rotor fingers by screws that are inserted in threaded holes of the rotor fingers. A retaining ring is connected to each end of the active part, to protect and withhold the rotor windings.
During operation, the zones of the fingers around the threaded holes can be damaged, because the material is exposed to cyclic loading and possibly fatigue due to the start up cycles.
The damage can include:                visible cracks, i.e. cracks extending over the surface of the rotor fingers,        non-visible cracks, i.e. cracks extending within the fingers but not over the surface thereof,        non-cracked fingers.        
During maintenance, in case of visible or non-visible cracks, the fingers are usually repaired by removing the damaged material and replacing it by welding new material to rebuild the removed parts of the fingers.
In case of non-cracked fingers, usually no actions are carried out. Nevertheless, in some cases, even if the fingers have no cracks when the maintenance is carried out, the material thereof is already damaged (for example by fatigue), such that cracks can be formed also a short time after the maintenance.